Summary of significant accounting policies (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Allowance for returns and price protection
Impact of a one percent change in the allowances for sales returns, price protection and other allowances on net revenues	$ 4
Advertising Expenses
Advertising expense included in sales and marketing expense	$ 401	$ 396	$ 343
Minimum
Revenue Recognition
Estimated service period over which revenues are recognized	5 months
Maximum
Revenue Recognition
Estimated service period over which revenues are recognized	1 year